I understand that X3 is certified for Win 7&8 but not Vista/XP. My system is running fine with Vista and I would not relish the thought of having to re-install multiple programs over many hours to upgrade to WIN 7 (I HATE Win 8!).
This leads to my question: Is there any technical reason why X3 would not run on my quad core machine with Vista, leaving aside the fact that I would give up technical assistance etc.?

You could try it but there is no support. I was a Vista user and am very glad to have moved to Windows 8. 

I have seen a few posts by users running X3 on Vista. There was a problem with the VST scanner on Vista that was fixed with X3c.

I used to have Vista as well, but now have Win8. If it's the Metro and UI that you don't like in Win8, you can use this free version of Classic_Shell to get the look of Win 7/Classic/Two Columns. I use this and it works very well. 
 
The main thing about Win8 is to make sure all your hardware has drivers for it.......Win8 also supports touch screen. :)

+1 What Mesh says.
 
Even though you may be satisfied with Vista, there are traps there. If not today, I would certainly consider moving to Windows 7 at the very least in the next few months. 3rd-party support for Vista is diminishing fast, and the sooner you move, the less frustrated you'll be when you have to later.
 
When Apple released Snow-Leopard, the slogan was, "The world's most advanced operating system. Finely tuned.". When Windows 7 came out, there was an underground slogan I read: "Vista...fixed."
 
I'm on 7 64-Bit now, and still love it. I haven't moved to Windows 8 yet. Have no incentive to at the moment. But Vista users do find compelling reasons to move up.

Brian's right. Though Vista for me was/is a great OS Windows 7/8 will impress you. I skipped Win 7 but I was impressed with Window 8's performance. Scook is also right. I use Classic Shell too.  But I also use the Metro start of Win 8.1.  You have a choice with Windows 8.1. 
 
 

I won't upgrade to 8 until my gear breaks :P

The least trouble you are going to have overall is by installing Windows 8.1, if you are going to upgrade I suggest stop thinking about it and doing it ;).
Go dual boot if you have to. Man up and Backup! No pain no gain!!